Processes so far known for preparing high purity saccharides of definite in chain length generally comprise decomposing a saccharide having an arbitrary chain length, typically starch or the like, with one or more appropriate amylases and separating the object saccharide from the other unwanted oligo- and/or monosaccharides by the per se known column chromatographic and other fractionation techniques [cf. e.g. Jiro Nikuni (chief editor): Denpun Kagaku Handbook (Starch Science Handbook), published by Asakura Shoten, Tokyo, 1987, page 452] followed, if required, by crystallization of the object saccharide to further purify the same from the other unwanted contaminant oligo- and/or monosaccharides [cf. e.g. Denpun Kagaku Handbook (vide supra), page 456.].
However, these processes invariably entail formation of unwanted byproduct oligosaccharides in addition to uncleaved dextrin and, therefore, are inherently disadvantageous.
As regards maltose, a process for producing high-purity maltose from terrestrial starch has been proposed (Japanese Kokai Tokkyo Koho JP 04-158795). However, this process is still unsatisfactory from the commercial viewpoint since high concentration charging of starch is impossible.
For solving these and other problems, there has been made an invention which is disclosed in the international patent application PCT/JP91/00984 (International Publication No. WO 92/01805). According to the above invention, a saccharide chain is transferred from a saccharide chain source, either directly or via an intermediate, to a substance which can be substantially separated from the desired saccharide of definite chain length (such substance is hereinafter referred to briefly as separable substance) by means of a transglycosidase, then the thus-obtained saccharide is treated with an enzyme capable of excising a saccharide chain having a specific chain length from said saccharide in an exo manner (such enzyme is hereinafter briefly referred to as exo-cleaving enzyme), and finally the desired saccharide of definite in chain length is isolated.
In this connection, a mode of causing an exo-cleaving enzyme to act on amylose is described by Nakakuki et al. [T. Nakakuki, K. Azuma and K. Kainuma, Carbohydrate Research, 128 (1984) 297-310] and the application of the periodate oxidation technique, among others, is described by Marshall et al. [J. J. Marschall and W. J. Whelan, Analytical Biochemistry, 43 (1971) 316-321]. However, these technologies have no direct relevance to the whole technology of the present invention.
Althogh the process described in PCT/JP91/00984 is an absolutely novel process capable of solving a number of problems, it still has the following disadvantages.
(1) Since a transglycosidase is employed as an essential reagent and accordingly a saccharide chain source must be provided for the supply of the necessary saccharide chain, the yield of the object saccharide is inevitably poor. PA0 (2) The use of a transglycosidase requires that said separable substance have a structure such that it can serve as a receptor for the transglycosidase (for instance, a substance whose hydroxyl groups in positions 2, 3 and 4 have a glucose-type configuration, or ascorbic acid or the like). PA0 (3) After the formation of a desired saccharide of definite chain length by the action of an exo-cleaving enzyme, a procedure is essential for isolating said saccharide and, therefore, the yield is necessarily limited. PA0 (1) The desired products can be obtained in very good yields since no transglycosidase is used. PA0 (2) Various saccharide materials, such as seed starch, root starch, chemically processed starch, dextrin, oligosaccharides, and mixtures of these can be widely used as starting materials. PA0 (3) Those high-purity products that cannot be produced by the prior art liquefaction/saccharification processes can be obtained, and that in improved yields. PA0 (4) The purification steps can be simplified. PA0 (5) Since starch after reducing end modification has a low viscosity, it is now possible to submit, on a commercial scale, high-concentration starch to the enzymatic reaction; this leads to improved productivity and cost reduction. PA0 (6) When, for instance, corn starch, which produces marked turbidity particularly at high concentrations, is used as the starting material, reducing end modification causes a decrease in turbidity and an increase in solubility so that the subsequent purification step can be simplified. PA0 (7) Such food additive as sodium hypochlorite can be used as the oxidizing agent for use in reducing end modification by oxidation. This is very suited for the production of desired products to be used in foods.
The object of the invention is to overcome the above technical drawbacks.